bp2build: add python_binary_host converter.

This CL adds a simple python_binary_host to native py_binary converter
for standalone modules that don't depend on libs. It also adds support
for the conditional py2/py3 build target based on the version prop.

Test: milestone-2/demo.sh full
Test: bazel query 'kind("py_binary rule", //bionic/...)'
Test: bazel run //bionic/libc/tools:genfunctosyscallnrs
Test: bazel run //bionic/libc/tools:genseccomp
Test: go tests
Fixes: 182236395
Change-Id: Ibe5ec6cd0dc12a61b3a449a8c723d80b891fae42